This invention relates to a process for preparing colourless chromogenic compounds which form coloured markings upon contact with acidic materials by electron donor-acceptor colour-forming reaction.
Particularly, this invention relates to a novel process for preparing triarylmethane derivatives represented by the following general formulae (I), (II), (III) and (IV): ##STR1## wherein each of R.sub.1 and R.sub.2 is at least one of hydrogen, halogen, nitro group, alkyl group, substituted alkyl group, amino group, substituted amino group, hydroxyl group, substituted hydroxyl group, thiohydroxyl group, or substituted thiohydroxyl group; each of R.sub.3 and R.sub.4 is hydrogen, substituted or unsubstituted alkyl group, cycloalkyl group, substituted or unsubstituted aralkyl group, substituted or unsubstituted aryl group, or substituted or unsubstituted unsaturated alkyl group, or one or both of R.sub.3 and R.sub.4 together with the adjacent nitrogen atom may form a heterocyclic ring; R.sub.5 is at least one of hydrogen, halogen, alkyl group, nitro group, substituted or unsubstituted amino group, substituted or unsubstituted hydroxyl group, substituted or unsubstituted thiohydroxyl group; R.sub.6 is at least one of hydrogen, halogen, lower alkyl group, lower alkoxyl group, amino group, lower alkylamino group, nitro group, phenyl group or phenoxy group; R.sub.7 is hydrogen, alkyl group, aralkyl group or phenyl group; and R.sub.8 is lower alkyl group or substituted or unsubstituted phenyl group.
There are known several methods for preparing triarylmethane derivatives represented by the general formula (I), for example, U.S. Pat. No. Re. 23,024 discloses a method in which the triarylmethane derivative is prepared from m-dimethylamino-benzoic acid and Michler's hydrol by condensation reaction. Another method for preparing triarylmethane derivative by condensation reaction of dimethylaniline with o-phthalaldehydric acid is described in "Beilsteins Handbuch der Organische Chemie", Vol. 14, page 549. However, these methods give triarylmethane derivative in low yields because a large amount of by-products is produced. Furthermore, these methods give only limited compound having a symmetrical structure.
The triarylmethane derivatives having the general formula (II) are novel compounds which are synthesized for the first time by this invention.
The known method for preparing triarylmethane derivatives represented by the general formula (III) and (IV) is disclosed in U.S. Pat. Nos. 2,443,092 and 2,597,965, "Beilsteins Handbuch der Organische Chemie", vol. 18, page 617 and Moriga & Oda, "Kogyo Kagaku Zasshi", vol. 64, page 1226, (1961), in which triarylmethane derivatives are prepared from dimethylaniline and phthalic anhydride by condensation. Another method is described in U.S. Pat. Nos. 3,491,112 and 3,491,116 and in "Beilsteins Handbuch der Organische Chemie", vol. 18, pages 618-619, in which benzophenone-2-carboxylic acid is first prepared from dimethylaniline and phthalic anhydride and then triarylmethane derivative is prepared from the resultant benzophenone-2-carboxylic acid and either dimethylaniline derivative or indole derivative.
However, these methods also give triarylmethane derivatives in low yields because of the production of a large amount of by-products. Particularly when substituted phthalic anhydride is used, a large amount of isomers is produced as well, therefore, the yield is more lowered. In addition, substituted phthalic anhydride has such a disadvantage that it is not suitable for industrial production because the process for the production thereof is complicated.
An object of the invention is to provide a novel process for preparing triarylmethane derivatives of a high purity in an extremely high yield.
Other objects and advantages of the invention will become apparent from the following description.